Percolation tanks are artificial recharge structures used to augment groundwater resources. Percolation tanks are simple earthen dams constructed across natural ephemeral streams, so that surface runoff is impounded in these structures. Due to evaporation losses, only a certain fraction of impounded water is subsequently expected to percolate through the tank bed and improve the yield of the downstream wells. However, scientific studies to evaluate the efficacy of Percolation tanks are very few. Water balance method (Raju, 1985) is generally employed for estimating tank contribution to groundwater augmentation. The water balance method takes into account the pan-evaporation measured at meteorological stations, rather than measuring it near the tanks. Such pan-evaporation data may not really applicable to large tanks and hence usage of such non-in situ (tank) measured pan-evaporation values in the evaluation of tank performance runs in to ambiguous results. On the other hand, tank contribution to groundwater could be qualitatively assessed by analyzing the stable isotope contents of tank water and groundwater (Nair et el., 1979) or injecting a tracer in to the tank and monitoring the tracer in the downstream wells (Nair et al., 1980). Thus the present invention offsets the ambiguity associated with the water balance and other tracer studies and provides quantitatively the contribution from percolation tank to groundwater.
Radioactive tracers used for such studies pose handling and hazardous problems. Hence evaluation of efficiency of percolation tanks warrants development of techniques making use of environmental tracers (such as chloride) present in rain water (tank water).
Reference may be made to the study by Raju (1985) [Raju, K. C. B. (1985). Recharge through percolation tanks and subsurface dykes, India, Proc. Semi. On “Artificial recharge of groundwater” held at Ahmedabad, India, p. 12C-1], wherein the study has evaluated the performance of percolation tanks using conventional water balance method i.e., the impounded water in the tank is equated to the sum of remaining water and the total loss of water from the tank. The total loss of water from the tank is the sum of evaporation and percolation. Here the evaporation value is taken from the meteorological station which is situated quite far away from the tank. The drawback of the study is usage of such non-insitu evaporation data for the water balance study which results in ambiguous estimates of percolation.
Another reference may be made to the study by Nair et al. (1979) [Nair, A. R., Pendharkar, A. S., Navada, S. V. and Rao, S. M. (1979). Groundwater recharge studies in Maharashtra: Development of isotope techniques and field experience. Isotope Hydrology 1978, IAEA-SM-228-240., II,: p. 803-826], wherein they studied the Shindawane percolation tank, Pune District, Maharashtra using δD of tank water, canal water and groundwater to evaluate tank influence area. The study segregated the wells into three groups based on their δD content. Group-I wells mainly recharged by the tank; Group-II wells getting feed from both tank and canal; and the Group-III mainly recharged by the canal. Thus the stable isotope study has provided qualitatively the influence of Shindawane percolation tank on the groundwater regime. The drawbacks associated with the stable isotope study are: the deuterium isotope measurement requires sophisticated and very costly equipment (about Rs. 150.00 lakhs) and moreover requires highly skilled manpower for handling stable isotope samples and measurements.
Yet another reference may be made to the study by Nair et al. (1980) [Nair, A. R., Jain, S. K., Rao, S. M., and Eapen, A. C (1980). Radiotracer technique to study the efficacy of Bangarwadi percolation tank. Proc. Workshop on “Nuclear Techniques in Hydrology”, held at Hyderabad, India, p.219-230], wherein radioactive tritium tracer in the form of tritiated water was injected in to the Bangarwadi percolation tank to evaluate its contribution to the groundwater and groundwater flow conditions. Their study could only identify the direction of the movement of percolating water towards irrigation wells in the down stream, but could not evaluate quantitatively the efficacy of the percolation tank. The drawbacks associated with this study are: usage of radioactive tracer which has inherent problems of safe handling otherwise health of the handling personnel is at risk; collection, transportation and measurement of tritiated water samples is to be carried out by trained and skilled personnel; and requires sophisticated and costly equipment for tritium measurement; disposal of the tritiated samples is a problem and one should take utmost care in this aspect and follow safety norms.